In a background-art single plate type color solid-state image sensing device represented by a CCD image sensor or a CMOS image sensor, a mosaic of color filters of R, G and B is mounted on an array of pixels (photodiodes) which perform photoelectric conversion. Color signals outputted from the respective pixels of the color solid-state image sensing device in accordance with the color filters are processed to generate a color image.
The color solid-state image sensing device provided with such a mosaic of color filters, however, has a disadvantage of low light utilization efficiency and low sensitivity because each of the color filters absorbs two thirds of incident light when the color filters are primary color type filters (of the three colors of R, G and B). There is another disadvantage of low resolution, especially, conspicuous false color because no more than one color signal (of a color) can be obtained from each pixel.
In order to overcome such disadvantages, therefore, an image sensing device having a stack of three photoelectric conversion layers has been researched and developed, for example, as described in JP-T-2002-502120 (corresponding to WO 99/39372) and JP-A-2002-83946. This image sensing device has a pixel structure in which, for example, photoelectric conversion layers for generating signal charge (electrons or holes) in response to blue (B) light, green (G) light and red (R) light respectively are stacked as three layers successively viewed from a light incidence surface. Moreover, this image sensing device has signal reading circuits which are provided in accordance with pixels respectively so that signal charges optically generated in the photoelectric conversion layers respectively are read independently. In the case of the image sensing device, almost of the incident light is subjected to photoelectric conversion so that utilization efficiency of visible light is almost about 100%. In addition, the image sensing device has such a structure that color signals of the three colors of R, G and B can be obtained in one pixel. Thus, there is an advantage that the image sensing device can obtain a good image with high sensitivity and high resolution (inconspicuous false color).
In an image sensing device described in JP-T-2002-513145 (corresponding to WO 99/56097), three layers of wells (photodiodes) for detecting optical signals are provided in a silicon substrate so that signals different in spectral sensitivity are taken out in accordance with difference in depth of the silicon substrate. That is, the image sensing device is designed so that a signal having a peak in blue (B) is taken out from a pn junction portion in the surface of the silicon substrate, a signal having a peak in green (G) is taken out from a pn junction portion in an intermediate portion of the silicon substrate and a signal having a peak in red (R) is taken out from a pn junction portion in a deep portion of the silicon substrate. Similarly to the image sensing device described in JP-T-2002-502120 and JP-A-2002-83946, this image sensing device can sense a good image with high sensitivity and high resolution (inconspicuous false color).
In the image sensing device described in JP-T-2002-502120 and JP-A-2002-83946, the three photoelectric conversion layers need to be stacked on the substrate, and it is difficult to form vertical wires by which pixel electrode layers provided separately in accordance with the photoelectric conversion layers and the pixels are connected to the signal reading circuits provided on the substrate, respectively. There arises a problem that the production process is complex to bring increase in cost and reduction in production yield.
On the other hand, in the image sensing device according to JP-T-2002-513145, separation of spectral sensitivity characteristics of the color signals of R, G and B is not sufficient as shown in FIG. 6 so that color reproducibility is poor. In addition, it is necessary to add and subtract output signals for obtaining true R, G and B signals. There is a problem that S/N is deteriorated by the addition and subtraction process.
An image sensing device described in JP-A-2003-332551 (FIG. 5) (corresponding to US Publication 2003/0209651) has been therefore proposed as an image sensing device to solve the problems belonging to JP-T-2002-502120 and JP-A-2002-83946 and JP-T-2002-513145. This image sensing device is of a hybrid type between the image sensing device described in JP-T-2002-502120 and JP-A-2002-83946 and the image sensing device described in JP-T-2002-513145. That is, this image sensing device is configured in such a manner that while only one photoelectric conversion layer having sensitivity to green (G) light is stacked as an upper layer on a silicon substrate, two pn junction portions (photodiodes) are provided in the silicon substrate in the depth direction so that a blue (B) light signal is taken out from the pn junction portion in a shallow portion of the silicon substrate and a red (R) light signal is taken out from the pn junction portion in a deep portion of the silicon substrate.
Since this image sensing device has only one photoelectric conversion layer, there is an advantage that a process for producing the image sensing device is so simple that increase in production cost can be suppressed and reduction in production yield can be almost avoided. In addition, the image sensing device is designed in such a manner that green (G) light is absorbed by the photoelectric conversion layer so that only blue (B) light and red (R) light can be made incident on the silicon substrate. Thus, separation of spectral sensitivity characteristics of the pn junction portion for blue light and the pn junction portion for red light in the silicon substrate can be improved, so that color reproducibility becomes good and an image with good S/N can be sensed.